Editor's Note: This update includes the views of Mars Rover
Principal Investigator Steven Squyres, who was not contacted prior to
publication of the initial version of the article.
The apparent discovery
of ancient salty bodies of water on Mars by NASA's Opportunity rover last year
is viewed as one of the most significant developments in planetary science.
But two new studies take a different view of
the data.
Rather than abundant surface water over
significant stretches of planet's history, as has been widely reported,
Opportunity's observations might represent the results of a meteor impact or
volcanic activity on an otherwise very dry world.
The counter arguments, presented in two
papers in the Dec. 22 issue of the journal Nature, go to the very heart
of the ultimate question about Mars: Was it ever warm and wet enough to support
life?
The new reports, however, are based on the
initial data from the Opportunity rover during its first 45 days on Mars.
The Mars rover mission's Principal
Investigator, Steven Squyres of Cornell University, said both of the new
studies could not take into account more recent data from Opportunity that were
not available to the researchers.
Multiple explanations
At Meridiani Planum, Opportunity found
photographic evidence of layered
sandstone that, when analyzed by chemical sensors, looked like it must have formed in the pryeesence
of significant amounts of water.
Today, Squyres told SPACE.com that
his team's interpretation has always been that the water was mostly
underground, sometimes seeping to the surface only to evaporate quickly.
"From the start, the media has overemphasized surface water and
underemphasized the underground water," he said.
In one of the new reports, scientists
conclude the deposits could be nothing more than volcanic ash altered by very
small amounts of acidic water and sulfur dioxide, which is a volcanic gas. The
research was done by Thomas McCollom and Brian Hynek of the University of
Colorado at Boulder.
"In our scenario, the water required to
support the chemistry in this bedrock would only have had to have been around
for months, years or perhaps as much as a few centuries," Hynek said
today. "This is very different than previous scenarios, which
require that a much larger amount of water be present for many millennia."'
"This scenario does not require
prolonged interaction with a standing body of surface water," the McCollom
and Hynek write.
The Meridiani region was probably more like
volcanic parts of Yellowstone, Hawaii or Italy than something like the Great
Salt Lake, McCollom said. "We think it was far less favorable for
past biological activity than other scenarios that have been proposed."
If McCollom and Hynek's scenario is correct,
its effect would be in "greatly reducing the possibility that these rocks
indicate that a habitable environment ever existed at Meridiani,"
according to Mark Bullock, a scientist at the Southwest Research Institute who
was not involved in the studies.
Or it could be ...
In the second paper, another group says an
impacting space rock can explain the chemicals and layered deposits observed at
Meridiani, as well as the infamous BB-sized spheres dubbed blueberries.
In fact, write Paul Knauth of Arizona State
University and colleagues, the blueberries are just too spherical and of
uniform size to be explained by formation in water.
Knauth's team proposes that the meteorite
generated a "ground-hugging turbulent flow of rock fragments, salts,
sulfides, brines and ice," leaving deposits that were later weathered by
small amounts of water embedded in the grains.
The scenario "can account for all of
the features observed without invoking shallow seas, lakes or near-surface
aquifers," the scientists contend.
There is little doubt that Mars, in its
early history, experienced bouts of intense flooding that involved water. The
evidence is plainly carved into the planet's surface in the form of canyons
bigger than any on Earth. But Bullock said those early episodes could have
involved very sudden and short-lived floods spurred by ice melting in meteorite
impacts that would have been frequent when the solar system was young.
"Both groups propose scenarios that
preclude the existence of significant bodies of water at the surface (at least
at Meridiani), and therefore that Mars may never have had conditions conducive
to life," said Bullock, who wrote an analysis of the work for Nature.
"This conclusion stands in sharp contrast to the provocative
interpretations that there must have been long-lived surface water to form the
Meridiani outcrops."
More Opportunity data
Squyres said a deeper understanding of the
situation came when Opportunity examined Endurance Crater, where observations
were made of 25 vertical feet of rock outcrops. Those results were published
just a month ago, after the two Nature papers had been submitted.
Knauth, McCollom and Hynek "hadn't seen
that stuff when they wrote their papers," Squyres said.
The nature of the layering and grain sizes
deeper inside Endurance Crater "is absolutely incompatible with a volcanic
or impact origin," Squyres said. It is "completely compatible"
with the idea of windblown material, and the upper meter or so "shows
evidence for deposition of water. The chemistry varies with depth in a way that
requires that subsurface liquid water interacted with the rocks after they were
deposited."
Squyres emphasized that his team has always
thought the water was mostly underground, occasionally creating small surface
lakes that evaporated quickly.
Squyres also stressed that nobody has done
anything other than good science with the data available. "It's always
good to have alternative hypotheses," he said. "In the end, the best
ideas win. It forces everybody to go back and sharpen their arguments. All of
this is a good thing."
It will take time for scientists to settle
this important debate. The outcome could effect decisions about where to send
future missions that would search for signs of life. Bullock called the
investigation vital, "whatever the ultimate verdict proves to be."
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